Alias declaration with invalid type panics on an unwrap

[mhasel]

Describe the bug
The following example will hit an unwrap in

rusty/src/validation/variable.rs

Lines 246 to 251 in bca3c31

	if let Some(ref initializer) = variable.initializer {
	let type_lhs = context.index.find_type(inner_ty_name).unwrap();
	let type_rhs = context.annotations.get_type(initializer, context.index).unwrap();
	validate_pointer_assignment(context, validator, type_lhs, type_rhs, &initializer.location);
	}

when the declared type does not exist.

To Reproduce
try to compile with --check:

VAR_GLOBAL 
    y: BOOL;
END_VAR

FUNCTION main 
VAR
    x AT y : SomeUndefinedType;
END_VAR

END_FUNCTION

Expected behavior
A diagnostic with Unknown type: SomeUndefinedType